Thermal time delay relay with large time constant



March 3, 1964 R. J. STRUBBERG 3,123,695

THERMAL TIME DELAY RELAY WITH LARGE TIME CONSTANT Filed Nov. 5, 1960 M5QYINVENTOR 1G4 ATTORNEY United States Patent 3,123,595 THERMAL TIME DELAY RELAY WITH LARGE Till TE QUNSTANT Ronald J. Struhberg, Bergeniield, Ml, assignor to Curtiss wright Corporation, a corporation of Delaware Filed Nov. 3, rats, gar. No. 67,137 lltl Claims. (Cl. Zed-1122) This invention relates to thermal time delay relays and in particular to relays of this type having a comparatively large time-constant.

Thermally actuated time delay relays designed for operation after a predetermined time setting have in general been a compromise between precision and uniform operation, involved space-consuming apparatus, and cost. This has been the case especially where the time delay was comparatively large, e.g. one to three minutes, and uniform, consistent and positive operation were required. Only where cost, mass, and space are not critical factors, have adequate relays of this type been available in the prior art.

The principal object of the present invention therefore is to provide a very compact, simple and inexpensive thermally operated time delay relay that is stable in both on and ofi" positions, and that is positive and consistently uniform in operation for a comparatively long time delay setting of the relay, such as for time-constants of from 60 to 180 seconds.

Another object of the invention is a relay of the abovedescribed type that can be easily fabricated, is of small size and mass, and that readily lends itself to applications where resistance to shock, vibration, and acceleration is essential.

The invention will be more fully set forth in the following description referring to the acornpanying drawing, and the features of novelty will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Referring to he drawing,

FIG. 1 is a partly schematic side view illustration of a thermal time delay relay embodying the present invention showing some of the elements in exaggerated space relation for descriptive purposes;

FIG. 2 is a side view of the relay as fabricated for cornpactness and ruggedness;

PEG. 3 shows the relay of PKG. 2 as viewed along the line 3-Zl of FIG. 2; and

FIG. 4 is an enlarged, detail view of the thermally actuated motive element of FIGS. 2 and 3.

Referring to FIG. 1, the motive element of the relay comprises a heat absorbing or thermal mass constituting a metal supporting strip 1 that is suitably fixed at its lower end to a base It. The strip 1 is of No. 302 or 304 stainless steel having both a high thermal coefficient of expansion and high tensile strength. Its upper end is attached to the upper end of a rigid, forcedransmitting metal member 3 that extends downwardly alongside the thermal strip 1. A thin pre-tensioned strip 4 of high tensile strength metal, such as Invar which has a low thermal coelficient of expansion, is secured at 4a to the lower or flee end of the force-t ausmitting member, also made of lnvar. The pre-tensioned strip 4 extends upwardly generally along the member 3 to the upper end of a comparatively massive and rugged compensator member 5 of 302 or 304 stainless steel where it is secured at 4!). The compensator member also is fixed at its lower end to the base .73 so that it and the interconnected parts between the two fixed base points form an integral structure that is zig-zag in form.

The thermal energy for operating the relay is provided by a heater diagrammatically indicated at 6 that surrounds or is in close heat-transfer relation to the heat storing or thermal mass 1. The upper end of the compensator d carries an electric switch contact 7 arranged to co-act with a companion contact 8 for controlling an external circuit, not shown. The contacts are of suitable contact metal such as silver or the like.

The operation of the relay is as follows: Assuming that the heater 6 is tie-energized and the motive element cold, the parts will be in the positions shown with the contacts 7 and 8 closed. It will be understood, of course, depending upon the external circuit requirements, that the contacts may be arranged at the opposite sides of the compensator so that the contacts are normally open or normally closed in the cold condition of the motive element; or fixed contacts may be positioned on both sides of the compensator for 2-pole operation, if so required. Although in the interest of brevity the contacts are shown in their simplest form, it w ll be apparent that movement of the compensator may be utilized to effect snap-action, as by a microswitch for example.

When the heater 6 is energized, the thermal mass, ele ment ll, absorbs heat and tends to expand in linear direction so as to move the member 3 upward. The timeconstant of the relay is dependent on the heat storage mass of element l, the dimensions thereof and the rate of heat transfer from the heater 6. Time-constant is defined as the time required for the movable contact to complete 63.2% of its travel. As the member 3 moves upward, the tension on the strip 4 is reduced so that the compensator 5, which is under continuous bending stress due to the pre-tensioned condition of the strip 4, tends to strai hten and move the contact 7 towards the left as viewed, thus opening the contacts. The contacts remain open as long as the heater is energized.

When the heater is ale-energized, the thermal element 1 contracts in linear direction as it cools, thereby forcing the rigid member 3 to its lower original position. This movement increases the tension on the strip 2- so as now to increase the bending moment acting on the compensator 5 and thereby warp it towards the right so as again to close the contacts. As the pre-tensioned strip a maintains a bending moment on the compensator at all times, positive operation of the contacts is assured. Contact chattering is thereby precluded.

FIGS. 2 and 3 show the assembled relay fabricated so as to be adaptable for enclosure in an evacuated glass envelope or the like. The lower ends of the elements It and 5 form flat terminal portions in and 5a respectively, that are separated by a spacer 9, the terminal structure so formed having a bore ill for suitably clamping or securing the relay unit to terminal base structure, not shown. As fabricated, the thermal element ll, member 3, and tension strip 4 are compactly arranged in generally parallel relation so that minimum bending stress is appiied to the force-transmitting member 3 and the space required for these elements is very small.

The heater 6, FIG. 3, comprises in the present instance resistance wire wound around and suitably insulated from the thermal strip 1, as by mica (not shown) so as to be in proper heat-transfer relation therewith. The thermal element may conveniently be of rectangular form for providing adequate heat-transfer area. The heater terminals 642 and 6b are for connection to a source of heater current, not shown.

As best shown in FIGS. 3 and 4, the heater 6 and thermal strip 3., hereinafter referred to in combination as the heater unit, are in effect telescoped within the member or yoke 3 which is formed as a channel member for strength and rigidity. Specifically, the yoke 3 comprises a rectangular frame, FIG. 4, having side flanges 3a forming the channel. The yoke has a rectangular opening 11 that forms cross-members 3b and 30 at the upper and lower ends thereof. The upper portion of the channel formed by the flanges 3a and cross-member 319 has fitted therein a T-shape member it: that forms the upper extension part of the thermal strip 1, the extension being rigidly secured as by a weld 1 to the yoke channel. The lower parts of the yoke and the heater unit are tin-con nected as indicated in KG. 1 so as to provide for transmission or movement to the compensator. The tension strip 4 is secured as by a weld dc to the lower cross-member 3c of the yoke so that vertical movement of the yoke in accordance with the expansion and contraction of the heater unit varies the tension on the strip 4- and thereby the bending moment on the compensator 5 as explained in connection with HG. 1.

As more fully described in copending application SN. 35,516, filed June 30, 1960, by Arthur L. Bastian, now Patent Number 3,076,881, granted Feb. 5, 1963, and assigned to the same assignee as the present invention, the comparatively rugged compensator 5 is dished along its longitudinal axis so as to provide a suitable degree of rigidity for opposing bending moments. The compensator has a coefficient of thermal expansion comparable to the thermal strip and is thus adapted to adjust the position of contact 7 in response to variation in ambient temperature to provide for temperature compensation in well-known manner. Where desired, the fixed contact (or contacts) may be adjustable for varying the overall operating time of the relay as described in S.N. 35,516.

it will be obvious that the thermal mass may be varied and thermal lagging may be applied at the heater unit to vary response time as required. ln practice, sample devices approximately l /4 inches in length and weighing less than /4 ounce embodying the present invention have een constructed to operate at time-constants of 60 to 180 seconds, depending on the thermal mass used.

It will now be apparent that the unique configuration and arrangement of the relay elements including the yoke assembly makes possible the use of a thermal time delay relay with large time-constant in a very small space. The zig-zag path through which the force is applied from the base through the thermal strip 1 and yoke to the tension strip for producing bending moment on the cornpensator lends itself to the use of a sufficiently large and properly supported thermal mass without increasing the overall length of the relay. Where the thermal element is not properly supported, an increase in thermal mass for increasing the time-constant increasesthe tendency toward vibration.

Furthermore, the pre-tensioned Zig-zag arrangement is in the case of large time-constants more efficient and stable than previous designs wherein lengthening of the compensator and thermal element to obtain adequate contact movement would introduce problems in flexibility, calibration, etc.

The above-described relay structure is highly resistant to shock, high frequency vibrations, and high acceleration forces so that it has ready application to many systems subject to dynamic influences.

it should be understood that this invention is not limited to specific details of construction and arrangement thereof herein illustrated, and that changes and modifications may occur to one skilled in the art without departing from the spirit of the invention.

What is claimed is:

1. A thermal time delay relay having a large time-constant comprising coacting switch contacts and means for controlling the opening and closing of said contacts including a heater, an elongated thermal element fixed at one end and in heat transfer relation to said heater, said element varying in length according to heating thereof, a force-transmitting member comparable in length to said thermal element connected to the other end of said thermal element and extending approximately parallel thereto in reverse direction to a point adjacent to said fixed end, and an ambient temperature compensator member having a thermal coefiicient or" expansion comparable to that of said thermal element and fixed at one end thereof so as to extend approximately parallel along the thermal element, said force-transmitting member and said compensator member having a connection under stress between their free ends respectively so as to be under initial tension and to subject the compensating member to bending moment whereby linear expansion and contraction or" said thermal element in response to heater influence varies the stressed condition of said connection and the bending moment applied to said compensator member for causing warping of the free end thereof in one direction or the other, said compensator operating said switch contacts, opening and closing of said coacting switch contacts being controlled by the aforesaid free end of the compensator according to the aforesaid Warping movement thereof.

2. A thermal time delay relay as specified in claim 1 wherein the com ensator member is subjected to con tinuous bending moment through the force-transmitting member and the warping end of said compensator memher is approximately opposite the non-fixed end of the thermal element.

3. A thermal time delay relay as specified in claim 1 wherein the force-transmitting member and compensator member are connected by a pretensioned thin strip of metal having high tensile strength for applying continuous bending moment to the compensator member.

4. A thermal time delay relay as specified in claim 3 wherein the pretensioned strip and force transmitting member have similar tlow thermal coeificients of expansion, and the thermal element and compensator member have similar high thermal coetlicients of expansion respectively'.

5. A thermal time delay relay as specified in claim 3 wherein the thermal element, force-transmitting member, pre-tensioned strip and compensator member are arranged in compact, approximately parallel relationship to each other with the corresponding ends of the forcetransmitting member and pre-tensioned strip connected near the fixed end of the thermal element, and the corresponding ends of the prc-tensioned strip and compensator member connected near the opposite end of the thermal element, so that the bending moment force is transmitted from the thermal element to the compensator member through a Zig-zag path.

6. A thermal time delay relay as specified in claim 3 wherein the force-transmitting member end that extends toward the fixed end of the thermal element is connected to the pretensioned strip at one end thereof, and the pretensioned strip extends in reverse direction from the force-transmitting member to the free end of the compensator member to which it is connected for applying bending moment to said compensator member.

7. A thermal time delay relay as specified in claim 6 wherein the thermal element is nested in the force-transmitting member in substantially parallel relation thereto, and the tension strip and compensator member are positioned adjacent and in approximately parallel relation to the force-transmitting member thereby to form a compact, pre-tensioned relay assembly of minimum length.

8 A thermal time delay relay as specified in claim 1 wherein the force-transmitting member has a channelshape form within which the thermal element and heater are positioned along the longitudinal axis of the channel.

9. A thermal time delay relay as specified in claim 8 wherein the aforesaid channel-shape member has an enlarged aperture to form a yoke, said heater and the heat transfer portion of the thermal element being positioned adjacent and opposite said aperture thereby to minimize heat trans er to said yoke.

10. A thermal time delay relay having a large timeconstant comprising a heater, an elongated thermal element varying in length according to heating thereof fixed at one end thereof and forming a heat absorbing mass in heat-transfer relation to said element, an electric switch, an ambient temperature compensator member controlling said switch and fixed on common base structure with said thermal element and closely spaced in approximately parallel relation alongside said element, a motion-multiplying and force-transmitting structure under stress disposed within the so-defined space and interconnecting the respective free ends of said thermal element and compensator member, said structure being under initial tension for applying a continous bending moment to said compensator member, said thermal element being adapted to expand and contract longitudinally in response to heater influence for varying the stressed condition of said structure and varying said bending moment and imparting in turn through the motion-multiplying structure corresponding Warping movement to the free end of the compensator member in one direction or the other, said electric switch controlled by the free end of said compensator member according to the Warping movement thereof.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A THERMAL TIME DELAY RELAY HAVING A LARGE TIME-CONSTANT COMPRISING COACTING SWITCH CONTACTS AND MEANS FOR CONTROLLING THE OPENING AND CLOSING OF SAID CONTACTS INCLUDING A HEATER, AN ELONGATED THERMAL ELEMENT FIXED AT ONE END AND IN HEAT TRANSFER RELATION TO SAID HEATER, SAID ELEMENT VARYING IN LENGTH ACCORDING TO HEATING THEREOF, A FORCE-TRANSMITTING MEMBER COMPARABLE IN LENGTH TO SAID THERMAL ELEMENT CONNECTED TO THE OTHER END OF SAID THERMAL ELEMENT AND EXTENDING APPROXIMATELY PARALLEL THERETO IN REVERSE DIRECTION TO A POINT ADJACENT TO SAID FIXED END, AND AN AMBIENT TEMPERATURE COMPENSATOR MEMBER HAVING A THERMAL COEFFICIENT OF EXPANSION COMPARABLE TO THAT OF SAID THERMAL ELEMENT AND FIXED AT ONE END THEREOF SO AS TO EXTEND APPROXIMATELY PARALLEL ALONG THE THERMAL ELEMENT, SAID FORCE-TRANSMITTING MEMBER AND SAID COMPENSATOR MEMBER HAVING A CONNECTION UNDER STRESS BETWEEN THEIR FREE ENDS RESPECTIVELY SO AS TO BE UNDER INITIAL TENSION AND TO SUBJECT THE COMPENSATING MEMBER TO BENDING MOMENT WHEREBY LINEAR EXPANSION AND CONTRACTION 